Parasitic capacitance refers to the undesired capacitance developed between nearby parts of an electrical component. Unfortunately, parasitic capacitance cannot be directly measured or modeled since any attempt to directly measure the parasitic capacitance of the electrical component with an instrument will indirectly induce some parasitic capacitance between the instrument and the electrical component. As such, what is measured instead is the performance affected by the parasitic capacitance. Imbalanced parasitic capacitance, for example, affects the AC common-mode rejection ratio (CMRR) performance of an amplifier. The CMRR is a ratio of the common-mode gain to the differential mode gain of the amplifier. The CMRR indicates how much of a common-mode signal applied at a pair of inputs at the amplifier will be rejected at the output. The CMRR includes a DC specification and an AC specification. At low frequencies, the AC CMRR is initially dependent on the DC performance. However, as the frequency increases, the AC CMRR performance begins to degrade. Further, for inputs with more imbalanced parasitic capacitance, the AC CMRR performance degrades faster than for those inputs with more balanced parasitic capacitance.
The effect of the imbalanced parasitic capacitance on the AC CMRR performance can be determined by comparing the output of the AC CMRR performance for a specific component to a preferred output. However, the effect of the imbalanced parasitic capacitance can only be measured after the integrated circuit, including the amplifier, is fabricated. As such, if the AC CMRR performance is not good enough, changes will likely need to be made to the layout of the amplifier circuit and the integrated circuit will need to be fabricated again. Such changes to the layout and integrated circuit result in an increase of time and money.
There is thus a need to efficiently balance the parasitic capacitances between the two inputs of the amplifier in order to improve the AC CMRR performance.